The worst thing is AMD is CPU designer,so probably has a clue of the microcode sizes,a few years ahead of launch,as they have to have functional design specs to make prototypes for testing,etc. So to then not make sure their reference platforms to have big enough BIOS chip,is either incompetence or the fact they expected to EOL these platforms earlier than they hinted at.
AMD then decided to hold off the B550 for 12 months. This is entirely on them!
If they knew they wouldn't release B550 for 12 months,they could have made another revision of B450 at the Zen2 launch which would have been certified to work with Zen3,ie,big enough BIOS chips,etc.
Called it B450X or something like that.